This invention relates to a transferred electron effect device and to a method of manufacturing such a device.
A Paper entitled `Advances in hot electron injector Gunn diodes` by H. Spooner and N. R. Coach Published in the GEC Journal of Research Vol. 7, No.1, 1989 at pages 34 to 45 describes a transferred electron effect device comprising a semiconductor body having cathode and anode contact regions, an active region of n conductivity type disposed between the contact regions and formed of a semiconductor material having a relatively low mass, high mobility conduction band main minimum and at least one relatively high mass, low mobility conduction band satellite minimum, and an injection zone adjoining the active region and defining a potential barrier between the cathode contact region and the active region for causing electrons to be emitted, under the influence of an electric field applied between the cathode and anode contact regions, from the injection zone into the active region with an energy comparable to that of a relatively high mass, low mobility conduction band satellite minimum of the active region.
As described in the aforementioned paper, and as discussed in detail in Chapter II pages 637 to 767 of the text book `Physics of Semiconductor Devices`, Second Edition by S. M. Sze Published in 1981 by John Wiley & Sons Inc. of New York, certain semiconductor materials such as gallium arsenide or indium phosphide exhibit a bulk negative differential resistance when an electric field above a threshold or critical field is applied across a sample of the material, allowing charge instabilities to grow to form accumulation or dipole layers. semiconductor materials exhibiting such a bulk negative differential resistance can, as first observed by Gunn, be used to form devices which generate a coherent microwave output when a dc electric field greater than the critical field is applied.
In order to enable electrons to be transferred to a relatively high mass, high energy low mobility satellite minimum (L) to obtain the negative differential resistance characteristic, sufficient energy has to be imparted to the electrons by the applied electric field. Conventionally, as discussed in the aforementioned paper, a transferred electron effect device comprises a relatively lowly doped n-conductivity type active region, for example an active region with a dopant concentration of about 1.times.10.sup.16 atoms cm.sup.-3, of an appropriate semiconductor material, for example gallium arsenide or indium phosphide, with relatively highly doped n-conductivity type regions being provided at opposed surfaces of the active region to enable ohmic contact to cathode and anode electrodes across which the electric field is to be applied. With such a conventional ohmic contact structure, electrons accelerated by the electric field do not achieve sufficient energy to transfer to a conduction band satellite minimum until they have traversed a given distance along the semiconductor body between the cathode and anode contacts. Thus, in such a conventional ohmic contact structure, the injection zone comprises a part of the active region and forms an acceleration zone in which electrons in the main conduction band minimum are accelerated and heated. Accordingly, the accumulation or dipole layers which result in the microwave oscillation grow some distance from the cathode and there is in effect a dead zone within the device. For a given applied electric field, the length of the acceleration zone is effectively fixed whilst the frequency of the microwave output is inversely proportional to the length of the device. Accordingly as demand occurs for devices capable of providing higher and higher frequency microwave outputs, the proportion of the length of the device taken up by the acceleration zone or dead zone increases adversely affecting device performance and efficiency.
In order to improve the power and efficiency of a transferred electron effect device with a given active region length, the accumulation or dipole layer should start as close to the cathode as possible. Accordingly, as discussed in the aforementioned paper, an injection zone or injecting structure is used to tailor the electric field, current and charge distribution at the cathode end of the active region so that electrons are emitted into the active region as hot electrons, that is electrons not in thermal equilibrium with the lattice, with an energy comparable to that of a relatively high mass, high energy, low mobility conduction band satellite minimum to enable the accumulation layer or dipole to start as close to the cathode as possible.
The aforementioned paper discusses in detail the use of a graded heterojunction injecting structure. That is the use of a region of, in this case of a gallium arsenide transferred electron effect device, an undoped linearly graded Al.sub.x Ga.sub.1-x As layer where x increases linearly from 0 to 0.3 over 50 nm (nanometers) giving an injection energy for the electrons emitted into the active region of about 250 meV close enough to the energy separating the conduction band main minimum (.GAMMA.) and a conduction band satellite minimum (L) for significant electron transfer.
This structure is produced by growing, on a highly n conductivity type doped gallium arsenide substrate, an epitaxial layer structure consisting of a highly n conductivity type buffer layer, then the relatively lowly doped transit or active region. The injecting structure is then grown as a graded bandgap heterojunction layer of Al.sub.x Ga.sub.1-x As with x decreasing away from the active region so that an abrupt interface is provided at the active region and finally a highly doped cathode contact region is provided on the injecting structure. As described in the paper, a highly doped depletion stop layer may be provided at the abrupt interface with the active region. The graded bandgap heterojunction injecting structure thus provides a potential barrier over which electrons are emitted into the active region with an energy comparable to that of a high mass, high energy, low mobility conduction band satellite minimum (L).
The actual devices or chips are formed from the thus-produced wafer by a standard integral heat sink (IHS) technology using conventional photolithographic and etching techniques which the paper states enables the heat sink region to be placed close to the epitaxial layers where most of the heat is generated, that is at the cathode end of the device.